Try to remember how back of your eyelids look, it's slightly different from complete darkness if you look carefully and it could be used as a good sign for when the dream ends, also you could try using mantra 'I will be still and will keep eyes closed when i wake up' or something like that as you fall asleep. Also don't think it's impossible, if you set something as impossible in your mind it will be harder to reach it, think positive!